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86119b0dba
Backward scan of a table wasn't implemented yet in MVCC so this achieves
that. I added simple test for mixed btree and mvcc backward scan but I
should add more intense testing for this.
<!-- CURSOR_SUMMARY -->
---
> [!NOTE]
> Implements backward scanning and last() in MVCC lazy cursor and adds
directional rowid iteration in the MVCC store, with new tests for mixed
MVCC+B-Tree backward scans.
>
> - **MVCC Cursor (`core/mvcc/cursor.rs`)**:
> - Implement `prev()` and `last()` with mixed MVCC/B-Tree
coordination using `IterationDirection`.
> - Add `PrevState` and extend state machine to handle backward
iteration.
> - Update `get_new_position_from_mvcc_and_btree(...)` to choose
rowids based on direction.
> - Integrate B-Tree cursor calls (`last`, `prev`) and adjust
`rewind`/rowid selection; tweak next-rowid when at `End`.
> - **MVCC Store (`core/mvcc/database/mod.rs`)**:
> - Add `get_prev_row_id_for_table(...)` and generalized
`get_row_id_for_table_in_direction(...)` supporting forward/backward
scans.
> - Add tracing and minor refactors around next/prev rowid retrieval.
> - **Tests (`core/mvcc/database/tests.rs`)**:
> - Add test for backward scan combining B-Tree and MVCC and an
ignored test covering delete during backward scan.
>
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<!-- /CURSOR_SUMMARY -->
Closes #3980